Generalize Decodable impl for arrays to all types

[panstromek]

This is almost the only impl that is not symetric with its Encodable counterpart. I tried to find out why in the history but it looks like this impl predates most of the generic symetric ones that are in this file, so it looks like it's not intentional.

I bumped into this randomly by changing a Vec to an array in some Mir types. The error was pretty confusing and It took me a while to figure out so I think it's worth generalizing this for somebody else in the future, even though it's technically not necessary at the moment.

We piggyback off of SmallVec impl to avoid adding a Default bound or MaybeUninit unsafe dance. This moves the assert to the loop.

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r? @mu001999

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[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

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[TIMING:end] tool::Tidy { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu } -- 0.000
fmt check
Diff in /checkout/compiler/rustc_serialize/src/serialize.rs:338:
 
 impl<D: Decoder, T: Decodable<D>, const N: usize> Decodable<D> for [T; N] {
     fn decode(d: &mut D) -> [T; N] {
-         SmallVec::<[T; N]>::decode(d).into_inner().ok().expect("invalid array length")
+        SmallVec::<[T; N]>::decode(d).into_inner().ok().expect("invalid array length")
     }
 }
 
fmt: checked 7059 files
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`